My research focuses on the depositional environments and conditions that preserve vertebrate fossils. By documenting the stratigraphic distribution of fossils in the field and by evaluating taphonomic features of fossil bones in museum collections, I interpret the sedimentological and biological processes through which fossils accumulated. My current research focuses on the preservation of mammal fossils in Miocene sequences of the western United States, in particular, the Barstow Formation in California.
Taphonomy
Taphonomy encompasses all processes that affect the remains of an organism, from death to final burial. By studying the physical features of fossils and geology of the fossil locality, it is possible to reconstruct these processes and the depositional environments that preserve fossils. I focus on the taphonomy of vertebrates and I study the processes that contribute to the accumulation of vertebrate remains on a variety of scales. At the locality scale, I look at features of fossil bones in order to interpret how animals died and what processes contributed to the accumulation of animal remains. Bone breakage patterns, tooth marks, weathering patterns, and abrasion are some of the taphonomically meaningful features that I assess in the field and in museum collections. In the field, I also document the geological aspects of fossil localities, including lithology, sedimentary structures and bed geometry, the number and type of specimens present, the spatial and vertical distribution of skeletal elements, and whether skeletal elements are articulated or associated. Describing these properties of a fossil locality are essential for interpreting its depositional environment.
At larger scales, the patterns of fossil occurrence over hundreds of meters can be important for understanding how landscape change affected the accumulation and preservation of fossils.
A fragment of a mandible from an extinct pronghorn (Merycodus necatus) that was chewed by rodents (white arrows).
Stratigraphic Paleobiology
Schematic representation of some of the factors affecting the preservation of fossils in the nonmarine rock record.
The stratigraphic distribution of fossils is dependent on the distribution of sedimentary facies in a stratigraphic sequence. Sedimentary facies represent the specific depositional environments inhabited by plants and animals in life or the depositional environments that are able to preserve their remains.
In the marine record, the relationship between facies and organisms is straightforward: the ecological tolerances of many marine invertebrates are often related to water depth, and so organisms live along gradients characterized by changing substrates and increasing water depth. As sea level fluctuates over time, organisms shift their ranges along the gradient, and the shifting depositional environments are preserved as stacked sequences of facies. Typical sequences of stacked marine facies form as a function of fluctuations in relative sea level. Therefore, fossil occurrences can be predicted in a stratigraphic sequence when these facies-stacking patterns are recognized.
In the nonmarine record, the relationships between organisms and habitat are less straightforward. Over regional scales, climate and tectonics are the dominant controls on geological processes, the distribution of ecosystems, and the geographic ranges of plants and animals within those ecosystems. The depositional environments that preserve the remains of plants and animals are much more restricted in extent, and are often highly localized. Recognizing the stratigraphic controls on fossil preservation in continental sequences is important for understanding the observed patterns of fossil distribution through time and space.
Paleoenvironmental reconstruction
As paleontologists, we are interested in the ecosystems inhabited by extinct plants and animals, and we use many tools to reconstruct the environments represented at fossil localities. Soil organic matter and soil carbonate preserve the isotopic signature of vegetation and are commonly used to reconstruct vegetation composition (in particular C3 vs. C4 vegetation), vegetation structure (open or closed canopy), and relative precipitation amount. Other indicators include phytoliths (microscopic plant fossils) and stable isotopes from carbonate nodules or biomarkers (plant-wax residues) that are preserved in sediment. Phytoliths are amorphous silica that is precipitated in the cells of plants, and many plants produce morphotypes that can be identified to family or functional type. The phytolith assemblage from a sediment sample can then be used to reconstruct the structure and composition of the vegetation growing at the time of deposition. Using these tools and methods throughout a stratigraphic sequence allows us to study how paleoenvironments changed throughout the depositional history of a geological formation.
Combining multiple approaches can yield robust reconstructions of paleoenvironments and the paleoecology of extinct animals and plants. Check out this example of the kind of information we use to study ancient environments.
In the Barstow Formation, California, I used geochemical and micropaleontological proxies to reconstruct vegetation composition, vegetation structure, and moisture dynamics through the Middle Miocene Climatic Optimum (MMCO), an interval of global warming that occurred 17 to 14 Ma. I used carbon isotopes in normal-alkanes (long carbon chains in plant leaf wax) and soil organic matter from sediment to reconstruct vegetation composition and moisture through the formation. I used phytolith assemblages collected throughout the formation to reconstruct changes in vegetation composition and structure through time.
Example of a phytolith assemblage from the Barstow Formation, California. Scale bar is 25 μm.
The Miocene Basin & Range
The Miocene (23.3 – 5.3 Ma) was a time over which many important geological, climatic, and ecological transitions occurred that helped shape the modern world. The Basin and Range physiographic province of North America formed during widespread extension and the collapse of a high-elevation region that occupied much of what is now Nevada, Utah, and Arizona. This widespread extension ultimately created numerous basins that contain records of the flora and fauna that inhabited the region as these basins developed through the Neogene. Mammalian diversity in western North America was very high during the Miocene and through this interval of heightened tectonic activity and the warm climate states of the MMCO.
As a postdoctoral researcher at the University of Michigan, I studied the role of landscape change and sedimentation history on the preservation of the fossil record of the Basin and Range over the Neogene. This work was funded by an NSF Collaborative Grant and conducted in collaboration with researchers at Stony Brook University, the University of Michigan, the University of Connecticut, and Purdue University.
The modern Basin and Range province (brown shaded region) extends across western United States and Mexico.
Around 35 million years ago, a high-elevation region existed in what is now Nevada and western Utah. Widespread extension caused this region to decrease in elevation and created the Basin and Range.